With the rapid spread of portable electronic equipment, the specifications required of the batteries used in such equipment have become more stringent with every year, and there is particular requirement for batteries that are compact and thin, have high capacity and superior cycling characteristics, and give stable performance. In the field of secondary batteries, attention is focusing on lithium nonaqueous electrolyte secondary batteries, which have high energy density compared with other batteries. These lithium nonaqueous electrolyte secondary batteries are winning an increasingly large share of the secondary battery market.
A lithium nonaqueous secondary battery includes: a negative electrode produced by applying a negative electrode active material mixture to both surfaces of a negative electrode substance including a copper foil in the form of an elongate sheet or the like, in the form of a coating; a positive electrode produced by applying a positive electrode active material mixture to both surfaces of a positive electrode substance including an aluminum foil in the form of an elongate sheet or the like, in the form of a coating; and a separator including a microporous polyolefin film or the like disposed between the negative and positive electrodes, and the negative and positive electrodes which are insulated from each other are wound in the form of a column or an oval to form a wound electrode body. In the case of a rectangular battery, a wound electrode body which is crushed into a flat form and in which negative and positive electrode current-collecting tabs are connected to a predetermined part of negative and positive electrodes, respectively, is accommodated in an outer packing in a predetermined form.
With respect to a 4-V-class nonaqueous secondary battery having a particularly high energy density among the lithium nonaqueous secondary batteries, as a positive electrode active material thereof, a material including a lithium compound oxide capable of reversibly intercalating and deintercalating lithium, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4 and LiFeO2 is used. As a negative electrode active material, carbonaceous materials, lithium or lithium alloys, metal oxides capable of intercalating and deintercalating lithium, for example, are used. Among them, particularly a negative electrode active material including a graphite material is widely used, since such a negative electrode active material has not only high safety, because while it has a discharge potential compared to a lithium metal or lithium alloys, a dendrite does not grow in it, but also such excellent properties as excellent initial efficiency, advantageous potential flatness and high density.
It is necessary that nonaqueous solvents (organic solvents) used in the above-noted nonaqueous secondary batteries have a high dielectric constant to electrolytically dissociate electrolytes and a high ion-conductivity in a wide range of temperatures. Examples of the nonaqueous solvents include organic solvents, for instance carbonates, such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylmethyl carbonate (EMC); lactones, such as γ-butylolactone; ethers; ketones; and esters. Particularly, a solvent mixture including EC and a noncyclic carbonate having a low viscosity, such as DMC, DEC and EMC is widely used.
However, in a nonaqueous secondary battery using the above-noted organic solvent, when a carbonaceous material, such as graphite and amorphous carbon is used as a negative electrode active material, an organic solvent is reductively decomposed on an electrode surface during a charging or discharging process and a negative electrode impedance is enlarged due to gas generation, deposition of by-products or the like, so that it is known that a disadvantage is caused wherein a charging and discharging efficiency is lowered and the cycle property is impaired, for example.
Thus, in related art, for suppressing a reductive decomposition of an organic solvent, various compounds are added to a nonaqueous electrolyte and for preventing a direct reaction of a negative electrode active material with an organic solvent, a technique for controlling a negative electrode surface coating (hereinafter, referred to as the solid electrolyte interface (SEI) surface coating) which is also referred to as a passivated layer has been important. For example, JP-A-8-045545 and JP-A-2001-006729 disclose a method comprising: adding at least one compound selected from the group consisting of vinylene carbonate (VC) and a derivative thereof into a nonaqueous electrolyte of a nonaqueous secondary battery; forming an SEI surface coating on a negative electrode active material by causing the above-noted additive to reductively decompose itself on a negative electrode surface before the insertion of lithium into a negative electrode by a first charging; and causing the SEI surface coating to function as a barrier for preventing the insertion of solvent molecules surrounding lithium ions.
JP-A-2001-006729 discloses a method in which for the same object as above, vinylethylene carbonate (VEC) or a derivative thereof is added into a nonaqueous electrolyte as an additive; JP-A-2001-202991 discloses a method in which for the same object as above, ketones are added; JP-A-2003-151623 discloses a method in which for the same object as above, an additive including VEC and at least one compound selected from the group consisting of VC, a cyclic sulfonic acid or cyclic sulfate ester and a cyclic acid anhydride is added; JP-A-2000-268859 discloses a method in which for the same object as above, a cyclic acid anhydride is added; and JP-A-2002-352852 discloses a method in which for the same object as above, an additive including VEC or a derivative thereof and a cyclic acid anhydride is added.